In conventional SLM projection systems, the entire device (modulator) is sequentially exposed to uniform colors for relatively long periods of time, with brief, spatially distributed transition periods between colors, called spokes (in reference to the physical spokes between filters on a color filter wheel), while the pulse width modulation (PWM) process is carried out for each respective color frame. FIG. 1 is an example illustrating this process, where the white illumination is passed through a color filter wheel 100, having red 102, green 104, and blue 106 primary color filter segments, producing sequential red 108, green 110, blue 112, red 114, etc. beams of light that expose the SLM. Other examples can include secondary color filters and/or include a white (clear) filter segment. These relatively long color periods with globally defined temporal boundaries allows the PWM bits to be turned ON and OFF either globally or phased by reset groups over the entire device load time. Since the PWM bits can be thought of as beginning and ending more or less simultaneously over the entire array, the PWM design process could be performed while treating the SLM as a whole, as long as certain design rules were followed.
The traditional PWM design rules developed over the years allowed a designer to size and arrange bit times to enhance performance and to adhere to predetermined bit weights. This meant that the designer could effectively place the resets that turn bits ON and OFF into the video frame timeline with confidence that, if the rules were followed, the loading of device data could be subsequently inserted into the same timeline without conflict. Furthermore, the designer would create one timeline for the entire array, knowing that the design rules would allow time phasing by reset group. However, although this illumination method is quite effective, two-thirds of the illumination is filtered out and lost at the color wheel, limiting the overall brightness of the projector.
With the introduction of the scrolling color recovery (SCR) optics method of illuminating a SLM, illustrated in FIG. 2, the concept of global bits is not longer applicable. This concept produces red 218, green 220, blue 222, and optional white color bands, which scroll across the SLM 216 so that multiple colors are applied to the SLM array simultaneously. This optical system is comprised of a light source 200 that supplies white light 202 to an integrator rod 204, with light from the integrator rod passing through a color filter wheel 206, through a condenser lens 214, on to the SLM 216. The filters are spiral shaped so as to produce three simultaneous red 208, green 210, and blue 212 bands of light. This concept also recovers a portion of the light that is not passed by a color filter segment by reflecting the secondary CYM light 224 back into the integrator rod 204 where it is recovered and sent back as RGB 226 light to the appropriate primary color filters.
Since the nature of the SCR method requires multiple colors to be applied to the SLM array simultaneously, what is needed is an electronic addressing method to accommodate the use of this illumination scheme in SLM devices. The present invention meets this need by dividing and controlling the SLM device at independent reset group levels, which are synchronized with the scrolling color bands. This method displays the bits in such a way that they exactly match the scrolling bands of color across the device and assure that the bits all add up in a way to provide PWM linearity. As a result, this approach provides a significant increase in the brightness in single-SLM projection systems.